Commonly owned U.S. Pat. application 07/487,350 filed Feb. 28, 1990, which is incorporated herein by reference, discloses the use of decanter centrifuges in a process for cleaning contaminated particulate solids. In particular, the above process is useful in offshore drilling operations where many tons of oil-laden particulate drill cuttings are generated by the drilling operation. In the process, the oil-laden particulate drill cuttings are mixed with a liquid solvent to dissolve the oil-based contaminant. The mixture of the particulate solids and contaminated solvent are then separated in a decanter centrifuge. The process further includes one or more repeated cycles of mixing with the solvent and separating the contaminated solvent in a decanter centrifuge. Once the particulate solids are fully cleaned they are discharged to the sea As such, the cost of storing and transporting the solids ashore for processing and disposal are avoided.
As with all systems on an offshore platform, size and weight are important considerations and, accordingly, it is desirable to minimize the size and weight of the system while fulfilling the operational requirements. One of the important factors in reducing the size of the system is the clarification (the completeness of the liquid/solid separation) obtained by the decanter centrifuge. It is generally known that the clarification obtained by a decanter centrifuge is largely affected by the size thereof, or more precisely, the residence time of the mixture being separated in the decanter centrifuge (a larger centrifuge provides a greater residence time for the same flow rate). Thus, enhancing the clarification by other means may allow for a smaller centrifuge. Furthermore, better clarification by the decanter centrifuge reduces the demand on the related devices in the system and allows for a reduction in the size of the related devices or of the decanter centrifuge itself. Accordingly, any enhancement of the clarification of the decanter centrifuge would provide a valuable weight and size reduction for the cleaning system.
Through investigation of the clarification of decanter centrifuges it has been determined that clarification is also dependent upon the mean particle size of the particulate solids. In other words, the larger the mean particle size, the more complete the clarification (all other factors remaining the same). Accordingly, the investigation for enhancing clarification developed into an analysis of the process for identifying the steps in the cleaning process where particles are likely to be broken, shattered or otherwise reduced in size.
One place in the process identified as a potential reducer of particle size is at the outlet of the decanter centrifuge. Referring to FIGS. 1 and 2 of the drawings, the particles are discharged in a conventional decanter centrifuge from a rotating drum 30 into a particulate receiving chamber 22. Due to the high rotational speed of the drum, the solids are flung from the discharge ports 35 and slammed to the walls of the particulate receiving chamber 22. The impact forces of such a collision are likely to cause the particles to disintegrate into smaller particles. The particles may further stick to the wall and a particulate receiving surface, such as a metal bracket or plough shown as 39 in FIG. 2, then strikes the particles and drives them around to the exit chamber 22. This is a severe impact and may lead to substantial break up of particles.
The disintegration of the particles would not present a problem when the particles are fully cleaned, but in the process disclosed in the above patent application, particles separated by one decanter centrifuge are subsequently delivered to another decanter centrifuge. If the first centrifuge reduces the particle size, the second centrifuge will obtain a reduced clarification. Thus, the clarification of subsequent centrifuges is likely to be enhance by minimizing the particle disintegration at the discharge of the decanter centrifuge.
Accordingly, it is an object of the present invention to enhance the clarification obtained by centrifugal separation devices.
It is a more particular object of the present invention to reduce the size and weight of systems for cleaning particulate solids.
It is a further object of the present invention to reduce particle disintegration at the discharge of decanter centrifuges.